The use of video imaging technology for monitoring conditions inside a furnace or boiler is well known in the prior art. In a typical application of such technology, a video camera is set up to monitor the condition of the boiler flame, so that adjustments to the fuel/air ratio and other operating parameters may be carried out. A display monitor provides an operator with a visual indication of the flame, as each parameter is adjusted.
In a related application of this technology, video imaging of the combustion area in a kraft pulp recovery boiler has proven useful in improving the boiler's operating efficiency. An example of such an imaging system is disclosed in U.S. Pat. No. 4,539,588 wherein a vidicon camera sensitive to light having a wavelength in excess of one micrometer is used for viewing the hot surface of a smelt bed in the bottom of a kraft recovery boiler. By monitoring the visual condition of the smelt bed, the operator can adjust the height of the bed and its configuration to achieve higher efficiency, faster turn-around following shutdowns, and greater boiler stability. The patent teaches that is is preferable to filter out visible light from the image, excluding all but a narrow band of wavelengths (70 nanometers wide) centered at 1,680 nanometers, to avoid interference from fumes and hot gases that overlie the smelt bed.
A problem in kraft recovery boilers not addressed by the above patent relates to the carryover of burning fuel particles into the upper portions of the boiler. Although this problem exists in other types of boilers to some extent, it is particularly troublesome in boilers used to recover pulping chemicals from the "black liquor" produced during the wood pulping process. Black liquor includes inorganic salts, e.g., sodium carbonate and sodium sulfate, mixed with wood pulp lignin and water, which are recovered by burning the black liquor (as a fuel) in a recovery boiler. Optimization of air distribution, fuel nozzle type, fuel distribution, percentage air/fuel and other parameters are important in the operation of such boilers to avoid an excessive quantity of the burning fuel particles from being carried into an upper portion of the boiler where they may plug passages in the boiler tubes, resulting in undesirable shutdowns and too frequent requirement for cleaning. In addition, the presence of excessive burning fuel particles in the upper portion of the boiler is an indication of incomplete fuel combustion, implying reduced operating efficiency. Often, the first sign of excessive burning particle carryover is an indication that the passages between the boiler tubes have become plugged, requiring shutdown of the boiler for cleaning.
The extent to which burning particles are being carried out of the combustion area of the boiler is difficult to monitor with prior art technology. Most conventional imaging systems such as that of the above-described patent are designed for monitoring conditions deep inside a boiler, and use a video camera with a respone time that is too slow to provide a clear image of the fast moving burning fuel particles. In addition, because of the greater depth of view and spectral sensitivity of such cameras, these particles are likely to be obscured by the background of the boiler interior.
The method most commonly used for checking the upper regions of the furnace for carryover particles is to conduct manual observations through an inspection port in the boiler wall. This procedure is hazardous to the operator due to the presence of noxious gases in the boiler, and because of the high temperatures to which the operator is exposed during the inspection. Consequently, such inspections are typically made on an infrequent basis, or not at all. Furthermore, such an inspection yields only a qualitative and very subjective evaluation of carryover conditions.
Clearly, an imaging system for monitoring the upper portions of a boiler for burning particles would avoid such hazards. However, even if such imaging systems were presently available, a video camera cannot directly quantitize the relative density of such particles, any more than a manual inspection. A quantitative assessment of the relative amount of burning particle carryover is required to properly evaluate the effect of adjustments made to the operating parameters of the boiler. Probes have been developed for monitoring carryover particles, but have not found widespread application, due to their requirements for maintenance, their cost, and the difficulty of interpreting the data they produce.
Accordingly, it is an object of this invention to provide an apparatus and method for imaging moving particles, minimizing the interference of noise, fixed background, obscuring fumes and hot gases, and varying illumination levels.
Another object is to quantitatively count the particles passing through a defined region of space.
A further object is to count fast moving particles using a video imaging device, the particles appearing in the image produced by the imaging device as streaks.
These and other objects of the invention will be apparent from the attached drawings and the disclosure of the preferred embodiment that follow hereinbelow.